Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern

Michael Ellerman
2013-10-08T04:33:30Z

On Wed, Oct 02, 2013 at 12:29:04PM +0200, Alexander Gordeev wrote:
> This series is against "next" branch in Bjorn's repo:
> git://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git
>
> Currently pci_enable_msi_block() and pci_enable_msix() interfaces
> return a error code in case of failure, 0 in case of success and a
> positive value which indicates the number of MSI-X/MSI interrupts
> that could have been allocated. The latter value should be passed
> to a repeated call to the interfaces until a failure or success:
>
>
> for (i = 0; i < FOO_DRIVER_MAXIMUM_NVEC; i++)
> adapter->msix_entries[i].entry = i;
>
> while (nvec >= FOO_DRIVER_MINIMUM_NVEC) {
> rc = pci_enable_msix(adapter->pdev,
> adapter->msix_entries, nvec);
> if (rc > 0)
> nvec = rc;
> else
> return rc;
> }
>
> return -ENOSPC;
>
>
> This technique proved to be confusing and error-prone. Vast share
> of device drivers simply fail to follow the described guidelines.

To clarify "Vast share of device drivers":

- 58 drivers call pci_enable_msix()
- 24 try a single allocation and then fallback to MSI/LSI
- 19 use the loop style allocation as above
- 14 try an allocation, and if it fails retry once
- 1 incorrectly continues when pci_enable_msix() returns > 0

So 33 drivers (> 50%) successfully make use of the "confusing and
error-prone" return value.

Another 24 happily ignore it, which is also entirely fine.

And yes, one is buggy, so obviously the interface is too complex. Thanks
drivers/ntb/ntb_hw.c

cheers